回複：ater softener 是除鈣鎂等硬離子的, 主要用於洗滌/洗澡 , 但除後的水含鈉高,非要用 reverse osm

Water softener is designed to remove hardness, which are calcium and magnesium. they do not create health problem, but does create scale in water pot, and waste detergent. some city provide water softener in their treatment plant, and the target water with total hardness in the range of 40-70 mg/l. but if water softener is not provided in the central water treatment plant, the concentration could be 200-300 mg/l or more. In this case, some family install home softener to remove calcium and magnisum. during the softening process, ion-exchange process is used, in other word, the softener adsorb calcium and magnesium and give out sodium in the water. for each 100 mg/l hardness, it probably produce same amount sodium. sodium is not harmful. as a matter of fact, the salt we eat is sodium chloride. the only problem people may concert about sodium in water is those who has diabetes. but for the amount produced during ion-exchange proces, it should be OK